phy dp83867: Fix compilation with CONFIG_OF_MDIO=m

When CONFIG_OF_MDIO is configured as module, the #define for it really
is CONFIG_OF_MDIO_MODULE, not CONFIG_OF_MDIO. So if we are compiling it
as module, the dp83867 doesn't see that OF_MDIO was selected and doesn't
read the dt rgmii parameters.

The fix is simple: Use IS_ENABLED(). It checks for both - module as well
as compiled in code.
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/dp83867.c

@@ -99,7 +99,7 @@ static int dp83867_config_intr(struct phy_device *phydev)
 	return phy_write(phydev, MII_DP83867_MICR, micr_status);
 }
 
-#ifdef CONFIG_OF_MDIO
+#if IS_ENABLED(CONFIG_OF_MDIO)
 static int dp83867_of_init(struct phy_device *phydev)
 {
 	struct dp83867_private *dp83867 = phydev->priv;